Some of the major properties of amino acids are as follows:
1. Amino acids are colourless crystalline water soluble but largely insoluble in organic solvents. Most α-amino acid have melting points near 300°C whereas their non-ionic derivative usually melt around 100°C.
2. Glutamic acid was the first amino acid formed during the amino acid synthesis. From this all other amino acids are formed by reductive deamination and transamination.
3. Stereoisomerism:
Glycine is the simplest AA where the R-group is a hydrogen atom. Except glycine, all amino acids are asymmetrical (optically active) and exist in two stereo-isomeric forms i.e. L- isomer and D-isomer. Only the a-amino acids are found in proteins, while D-amino acids are rare in nature. Recently 2 free D-amino acids found in mammals, i.e. D-serine in the forebrain and D-aspertate in brain and in PNS.
4. Ionization:
Amino acids have two ionizable groups i.e. a-amino and a-carboxyl groups. In addition to these, some amino acids have additional ionizable side-chain group (Asp, Glu, Arg, Lys,). In neutral solution (pH=7.0), amino acids are predominantly zwitterions or dipolar ions, where the α- amino group is protonated (-NH3+) and the a-carboxyl group is dissociated (-COO–).
Therefore, a- amino acids are ampholytes (amphoteric electrolytes) i.e. can act as both acid (protein donor) and base (proton acceptor). In acid solution (e.g., pH1), only the a-amino group is ionized. In alkaline solution (e.g., pH1), only the a-carboxyl group is ionized. The pH at which an amino acid bears no net electric charge i.e. exist in form of Zwitter ion is called pi or isoelectric pH. pI can be calculated as the midpoint between the pK1 and pK2 values.
5. pK value :
All amino acids possess at least two weakly acidic functional groups. However, – COOH is several thousand times stronger acid than – NH3+. The pK value is the pH at which these acidic groups are half dissociated. The pK ot- COOH group and – NH3+ group are respectively called pK1 and pK2. In all the 20 standard amino acids, the value of pK1 varies from 1.8-2.9 and the pK2 value varies from 8, 84 -10.78. Aromatic amino acids like tryptophan, tyrosine, histidine and phenylalanine absorb UV-light.
6. Peptide bond formation:
The peptide or amide bond (CO-NH) is a covalent linkage formed by removal of one water molecule between the a-amino group of one amino acid and the a-carboxyl group of another amino acid. After the formation of peptide bond, each amino acid is known as amino acid residue.
The peptide group is rigid and planer, while the peptide bond has partial double bond character with no rotational freedom. A peptide is a molecule that consists of two or more amino acid residues, e.g., dipeptide (2 amino acid residues), tripeptide (3 amino acid residue) etc. A polypeptide usually contains 20-4000 residues.
The backbone of the polypeptide includes the repeating sequence of carbon atoms and peptide bonds while the side chains of different amino acids responsible for functioning of the protein. A polypeptide with more than 100 residues is generally called a protein. A protein may consist of one or more polypeptide. A peptide is a polar molecule starting with the amino terminus (N-terminus) and ends with the carboxy terminus (C-terminus).